How do you read a ruler in mm?

How long is that line? How wide is that suitcase? Will that box fit on this shelf? Questions like that are one of the few things you can't necessarily Google an answer to nowadays. But they can be solved with the aid of a simple measuring tool, the ruler, or its grown-up cousin the yardstick. While some rulers will show only the US customary measures of inches and feet, most also have a metric side that measures length in millimeters and centimeters.

Choosing the Right Ruler Measurements

Before you start lining your ruler up and measuring things, take a look at the numbers running down the sides. If there are numbers on just one side of the ruler, they almost definitely show US customary measures: inches and feet. If the ruler has measurements down both sides, one side will show US customary measures; look for the side where the larger marks (inches) are numbered up to 12.

The other side of the ruler will have markings for cm and mm measurement. The distance between numbered marks on that side will be shorter than on the inches side, and the numbered marks will go as high as 30, because there are approximately 30 centimeters in 12 inches (the length of the standard ruler). The small lines between the larger, numbered lines represent millimeters.

Line It Up

Now that you've identified which side of the ruler has metric measurements, line that side of the ruler up with the object you're measuring. The "zero" line on the ruler usually doesn't line up exactly with the edge of the ruler, so make sure you put that zero line even with one edge of what you're measuring.

Tips

• Millimeters are usually used to measure very small things. If you're measuring a very small object, it may be easier to bring the object to the ruler instead of the other way around.

Read Your Millimeter Ruler

Now that the "zero" mark on your ruler is lined up with one edge of the object you're measuring, read along the ruler until you reach the far edge of the object being measured. Because the marks on a mm ruler are quite small and aren't numbered, it may help to put your finger, or the point of a pen or pencil, down to help you keep your eye on the correct mark.

Next, count the number of millimeter marks, starting from the zero line of the ruler and continuing until you reach the mark that lined up with the far edge of your object. The number of marks equals the object's measurement in millimeters.

Converting From Centimeters to Millimeters

You don't actually need to count every single millimeter mark along the ruler – you can use the numbered centimeter marks as a shortcut. Each centimeter is equal to 10 millimeters, so if your object measures 4 centimeters long, that's equivalent to:

4\times 10=40\text{ millimeters}

Often, your measurement in millimeters will fall between centimeter marks on the ruler. In that case, count centimeters up until the mark just before your measured object, then add in however many more millimeter marks it takes to reach the line you measured to.

Once you understand that 1 centimeter is 10 millimeters, you don't need to do the multiplication to convert between these two units of measure. Just count by tens for each centimeter mark.

For example, if you'd measured an object that reaches up to the 5 centimeter mark and then another 5 millimeters beyond that, instead of counting "one... two... three... four... five..." for the centimeters and then multiplying to convert them into millimeters, you can just count millimeters by tens: "ten... twenty... thirty... forty... fifty..." and then add in the remaining 5 millimeters for a total measurement of 55 mm.

Rulers are a great tool for learning about, and taking, real-world measurements in a variety of units. Most rulers in the United States have markings on two sides: One side of the ruler has lines for measuring inches and feet, while the other side has lines for measuring millimeters and centimeters. The lines on the centimeter ruler side will be closer together than those on the inches/feet side. The smallest marks on that side, which denote millimeters, are so close together that they aren't numbered – but you can still count them.

Start With Ruler Counting

Before you actually start measuring things with your ruler, the first step is to master the process of ruler counting. Locate the zero end of the ruler, and then count each individual mark along the edge of the ruler. Each mark represents 1 millimeter or mm, so counting five marks is the same as counting 5 millimeters, counting 10 marks is the same as counting 10 millimeters and so on.

Start Measuring With Your Ruler

Once you're comfortable with ruler counting in millimeters, it's time to transition to taking actual measurements. Keep in mind that because your ruler is straight, it can only be used to measure straight distances.

Place the flat end of the ruler against whatever it is you're measuring, and line the zero mark on the ruler up with one end of the object to be measured. Next, starting from the zero end of the ruler, count the marks along the ruler just as you did for "ruler counting."

Once you reach the other edge of the object you're measuring, stop. Remember that however many marks you've counted along the ruler equal the number of millimeters you've measured. So if you counted 23 marks to reach the far end of the object, it's 23 millimeters long; if you counted 46 marks to reach the far end of the object, it's 46 millimeters long; and so on.

Tips for Taking Ruler Measurements

Although you could just count millimeters along your ruler from one end to another, there's an easier way. Instead of counting every millimeter, count the larger marks (on the same side of the ruler) that represent centimeters, until you're close to – but not past – the edge of whatever you're measuring. Multiply the number of centimeters you counted by 10, and then keep counting millimeters from there.

Why does this work? It's because each centimeter is the same as 10 millimeters. (You can confirm this by counting the millimeter marks in between centimeter marks on your ruler.) So when you count centimeters, it's like counting by tens in millimeters. Multiplying the number of centimeters by 10 converts the measurement back into millimeter form. If you want to skip that step, you can also just count by tens, adding an extra 10 for each centimeter mark you pass on the ruler.

Counting the Centimeter Marks

If whatever you're measuring is more than 10 or 20 millimeters long, you may be asked to measure it in centimeters instead. You can do this by counting along the ruler's centimeter marks as just described. Once you reach the centimeter mark nearest the edge of your object (but not past it), stop and write down how many centimeters you've counted, followed by a decimal point.

Next, count how many millimeter marks remain between the last centimeter mark and the edge of your object being measured. (The result will be 9 or fewer – if you counted up to 10 millimeters, you'd reach the next centimeter mark.) Write this new number down to the right of the decimal point. The answer will be your measurement in centimeters.

So, for example, if you measure an object that's 4 centimeters long, and then another 3 millimeters, its ultimate length is 4.3 centimeters long.

Tips

• This works because each millimeter is equal to 1 centimeter, so each millimeter mark you count along the ruler is equal to 0.1 centimeter.

Converting Between Millimeters and Centimeters

Last but not least, if you have a measurement in millimeters but want to compare it to other measurements taken in centimeters, you should convert the millimeter measurement into centimeters. (That way you can compare similar units – it's like comparing apples to apples, instead of trying to compare apples to oranges.)

To convert from millimeters to centimeters, divide your measurement by 10. So for example, if you measured 52 millimeters along the ruler, you'd divide by 10 to get the equivalent in centimeters:

\frac{52\text{ mm}}{10\text{ mm/cm}}=5.3\text{ cm}

Did you notice that you can get the same result by simply shifting the decimal point one space to the left? That gives you the same result as dividing by 10.

If you want to convert from centimeters to millimeters, do the reverse: Multiply by 10 or, for a handy shortcut, move the decimal point one spot to the right. For example, if you had measured something that was 7.9 centimeters long, you could multiply that by 10 to get the result in millimeters:

7.9\text{ cm}\times 10\text{ mm/cm}=79\text{ mm}

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